Whether of small or large caliber, the rifled barrel has dominated projectile weapons as a means of introducing flight-stabilizing spin to the projectile. Rifled barrel twist rates from 1:36 or less to 1:7 or more produce rotational rates of from 30,000 to 300,000 RPM or more, which is sufficient to stabilize projectiles with length to diameter ratios of up to 5:1 or more. At any caliber or ratio, such projectiles from un-rifled barrels readily tumble, which greatly reduces accuracy and range.
The use of single-projectile bullets or slugs with smooth-bore shotguns has a long history of innovation to increase range and accuracy. U.S. Pat. No. 3,726,231 discloses a waisted slug known as the BRI slug or bullet. Such waisted slugs grew to prominence in the 1970's and 1980's. The hollowed aft portion of the Foster slug improves accuracy by placing more mass in the front of the projectile, therefore reducing to some degree the tumble of solid slugs. The Brenneke slug achieves a similar result by retaining connection with the lower density wadding during flight, thereby increasing range and accuracy in smooth bore guns.
The late 20th century saw increased interest in use of the shotgun slug, which was motivated in part by a combination of user preference and regulatory mandate, especially on relatively flat terrain and in densely populated areas. The availability of rifled shotgun barrels also increased; however, this enhanced slug performance came at much higher cost for the shotgun and at the expense of unsatisfactory bird and buck shot performance; bird or buck shot from a rifled barrel can produce a hollow ring pattern. In addition, any gun with a rifled barrel may not be legally classified as a shotgun in some districts; therefore, prohibiting their use for hunting.
Nevertheless, there have been ongoing development efforts in sabot-shuttled projectile technology intended for firing through rifled barrels. U.S. Pat. No. 5,214,238 discloses a sabot for chambering conventional or sub-caliber bullets in a rifled shotgun. Sub-caliber bullets from less than .22 caliber to greater than .50 caliber have been disclosed. U.S. Pat. No. 5,415,102 discloses a muzzle loading sabot-shuttled bullet. These sub-caliber rounds provide higher kinetic energy, range, and accuracy over full-sized slugs.
Sources of projectile inaccuracies include wind, the effect of gravity during long flight times, and variations in gun powder charge and drag. Drag causes bullet velocity to decrease, which increases the time of flight to a target and the subsequent effect of gravity and wind. Slugs and full-caliber projectiles are especially subject to drag because of their relatively large diameters.
The types of drag that act on transonic and hypersonic bullets are from aerodynamic shock waves, skin friction, and base vacuum at the back of the projectile. U.S. Pat. No. 5,297,492 discloses a sabot shuttled sub-caliber projectile having a fin stabilized sub-caliber projectile further comprising an internal blind core filled with a tracer or propellant composition in part to reduce base vacuum drag at the rear of the projectile. Fins increase drag, but this is offset by the smaller diameter projectile. Sub-caliber, sabot-shuttled bullets without fins but fired through rifled barrels demonstrate comparable extended range and accuracy.
Bullet or projectile shape has a predictable effect on range, and ideal shapes and dimensions cannot be used without either a sabot or rifled barrel or a sabot with a finned projectile, FIG. 1 shows a comparison of drag characteristics for various nose shapes in the transonic to low Mach regions. The rankings shown in FIG. 1 are: superior (1), good (2), fair (3), and inferior (4). As illustrated in the graph shown in FIG. 1, the aerodynamic Von Karman and ¾ Parabola shapes have the lowest drag at transonic and hypersonic velocities. See, Gary A. Crowell, Sr., The Descriptive Geometry Of Nose Cones (1996). The cone profile disclosed in U.S. Pat. No. 5,297,492 has poor performance at velocities between Mach 0.8 and Mach 2.2. Mach 1.0 in dry air at 68° F. is 1,125 feet per second, and shotgun slug muzzle velocities typically range between Mach 1.2 and 1.8.
In U.S. Pat. No. 6,085,660, Campoli et. al. disclose a cannon sabot that allows “flow of a portion of the gas through the sabot” partially transverse to the barrel axis to counter the rotation caused by the rifling for finned projectiles or causing a low speed rotation of up to 6,000 rotations per minute from a smooth bore. This reduced rate of rotation helps stabilize finned projectiles, but it is insufficient to stabilize non-finned projectiles.
Regardless of caliber, there is a need for full caliber and sub-caliber non-finned ammunition that can spin-stabilize projectiles when fired through a smooth bore.